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Vaccines are increasingly formulated with antigens consisting of subunits of microbial pathogens generated through chemical processing or genetic engineering to ensure the safety of vaccines. Unfortunately, these vaccine antigens are poorly immunogenic and adjuvants are added to stimulate an effective immune response. Adjuvants work, at least in part, by increasing the antigen uptake and by promoting the activation of dendritic cells (DCs), a critical step in the initiation of the immune response. The most widely used adjuvants in human and veterinary vaccines are aluminum-containing adjuvants which generally induce a good antibody response, have an excellent long term safety profile, and are relatively inexpensive. However, aluminum adjuvants are ineffective in inducing a cell-mediated immune response; are inactivated by freezing; can have a detrimental effect on the stability of vaccine antigens; and are associated with local adverse vaccine reactions. In addition, aluminum is not biodegradable, and most of it is excreted via the kidneys and sweat glands. New adjuvants need to stimulate the appropriate immune response, but the single most important consideration is safety. In addition, adjuvants need to be biodegradable, compatible with various antigens, and inexpensive.